Caption: Lenticular cloud. This cloud type, also known as altocumulus lenticularis, gets its name from its characteristic smooth lens-like shape. These clouds form when moist air flows over a mountain, creating a series of waves on the downward (lee) side. If alternate layers of moist and dry air are present, the clouds form a stack. These clouds are often mistaken for Unidentified Flying Objects (UFOs). Photographed in Colorado, USA.